1. Field of the Invention
The present invention relates to drop detectors for detecting particles or liquids that are propelled toward and adhere to substrates and, in preferred embodiments, to a method and apparatus for detecting drops of a jettable liquid (such as ink) ejected from an ink jet apparatus onto a substrate, based on heat content of the liquid drop.
2. Description of Related Art
Various approaches have been considered for identifying drops of ink ejected from an ink jet apparatus. Such approaches include sensing the impact force of drops on a mechanical structure, interrupting a beam of light by drops of ink, sensing differences in the drive waveform, measuring the mass build up on a target, and observing changes in electrical charge as a drop is ejected.
For example, U.S. Pat. No. 4,323,905 to Reitberger, et al, describes an example of an impact force sensing device for detecting the presence of ink droplets during the ink jet printing operations. The impact sensing device comprises a foil having a metal layer which is placed over a counter electrode. A voltage is applied to the electrode and the metal layer. The force of an ink droplet impinging on the foil momentarily deflects the foil and causes a change in capacity which in turn causes a voltage change at the electrode, whereby the presence of the ink droplet is detected.
U.S. Pat. No. 4,835,435 ('435 patent) describes another impact force type drop detector that produces an output signal with a selected resonant frequency when the detector is struck by a drop. The drop detector has a piezoelectric membrane mounted to a substrate. When a drop strikes the piezoelectric membrane, the membrane vibrates at the selected resonant frequency. The vibrations of the membrane produce an output signal having a frequency equal to the selected resonant frequency. However, with these impact type drop detectors, which rely on deflection or vibrations of a very sensitive membrane, it can be difficult to isolate the vibration caused by a drop of ink from acoustic or other vibrations caused by background noise.
Another prior art approach to drop detection uses optical devices. Such approaches typically employ an emitter for directing a collimated beam of light at a photodetector. When a drop travels through the light beam, the photodetector output varies to thereby indicate the detection of a drop. However, the emitter and the photodetector in such systems must be precisely aligned so that drop trajectory would fall within the collimated beam of light. The precise alignment of the optical system is relatively difficult and subject to mechanical failure.
Typically, in order to detect drops from large arrays of jets at the same time, these prior art drop detectors would tend to become substantially large in size, precluding some compact ink jet apparatus designs. Alternatively, for smaller sized prior art drop detectors to detect drops from large arrays of jets, the jet array or the detector must be moved so that each jet could be tested. As a result, the process to determine whether or not all the jets are normally operating can be relatively time inefficient and can require relatively complex mechanical movements.